The present invention relates to ink jet printer systems for reproducing and enlarging color images. A scanner electronically scans an original color image to develop electrical signals representing that image which are then used to control the paint spraying of the larger duplicate image. More particularly, but not by way of limitation, the ink jet printer of the present invention is a large format printer designed for producing color reproductions on an imaging medium for numerous interior and exterior uses including, but not limited to: signs, fleet graphics, backdrops, illuminated panels, architectural displays, and billboards.
With the development of scanning techniques capable of accurately reducing a color image to a series of electrical signals, systems using those signals to reproduce photos, pictures, and the like into enlarged images for use on signs, billboards, etc. have been developed. One such system is U.S. Pat. No. 3,553,371 issued on Jan. 5, 1971 to Suenaga. The Suenaga patent discloses a method for producing an enlarged multi-colored print by scanning an original picture. Electric signals representative of that picture are produced and used to control the rate of discharge of ink from a group of spray nozzles. Each electrical signal corresponds to a pixel and represents the density of ink to be applied to the paper. The electrical signals vary ink density by controlling both the amount of nozzle opening and the flow rate of compressed air past the nozzle. As the nozzles are opened, the variable stream of compressed air flows past the nozzle, thereby picking up the ink and applying it to the paper.
The above method of paint density control experiences problems in ink application. Densely colored pixels require increased ink flow, accomplished through large nozzle openings and increased compressed air flow rates. The expelled ink strikes the paper with sufficient force to cause a paint mist to form which settles back onto the paper in either previously painted or unpainted pixels. The misted ink on the paper may result in noticeable marks on the finished image.
Additionally, the mist settles on the nozzles, resutling in a coloring problem. Excess ink on the spray gun nozzles cause extra ink to be applied to the paper during subsequent spray cycles. The excess ink changes the color density of subsequent pixels. That is, the actual color of the pixels is incorrent from the desired color, thus changing the color of the entire image. The color changes are noticeable to the human eye and result in a reproduced image of poor quality.
An attempt to overcome the problems encountered in the prior art is disclosed in U.S. Pat. No. 4,914,522 issued Apr. 3, 1990 to Duffield, et al. The '522 patent scans a color image to produce control signals representative of the density of the color to be applied to an imaging medium. The '522 patent uses the developed control signal to operate four spray heads which spray ink onto the imaging medium to a desired density. However, unlike Suenaga, the '522 patent produces the desired ink density by modulating the amount of time the ink is applied to the imaging medium rather than modulating the intensity of the ink flow. The '522 patent attempts to solve the misting problem encountered by Suenaga by delivering the ink for longer periods of time under reduced pressure. Although the ink strikes the imaging medium at a reduced pressure, some misting is unavoidable, which results in ink accumulation on the spray head nozzles.
The '522 patent attempts to remove the excess ink by providing a continuous air flow about the nozzles of the spray heads. However, the design of the '522 patent is such that the ink accumulation is not prevented. The '522 patent does not solve the ink accumulation problem because it uses a single constant air pressure source. The single constant air pressure source applies the ink onto the imaging medium with sufficient force to cause misting, but is of insufficient force to prevent the ink from accumulating on the nozzles. The excess ink changes the density of the applied color, thereby changing the overall color of the image as described above.
Furthermore, because the air flow is insufficient to clear the nozzles, they will clog up and cease to function before printing of the entire sign is finished. Once the ink nozzles clog up, they must be cleaned, which is a labor intensive project, extremely wasteful of time. Although cleaning is a problem, the major concern with the '522 patent is that the image cannot be produced in one continuous print. If a reproduced image is not produced in one continuous print, color variations occur which are noticeable to the human eye. That is, incorrect color densities occur which result in the production of incorrect color shades. Thus, the system disclosed in the '522 patent is incapable of producing an enlarged image having the desired color scheme.
As a result of the difficulties encountered in the '522 patent, an alternative design of the spray head was adopted. That design is embodied in U.S. Pat. No. 4,999,651 issued on Mar. 12, 1991 to Duffield, et al. In the '651 patent, the continuous air supply about the nozzles to prevent ink accumulation was eliminated. Instead, housings positioned about the nozzles to minimize the deleterious ink accumulation on the nozzles are provided. The housings operate to shield the nozzles from the mist that results from the ink striking the imaging medium. Although the housings help to reduce the ink accumulation on the nozzles, they do not eliminate it. To overcome the ink accumulation problem, the '651 patent cautions system users to clean the nozzles and the face of the housings at reasonable intervals. Thus, the system of the '651 patent is incapable of producing an image in one continuous print, which again results in incorrect ink densities. Images produced using the '651 patent will have noticeable color flaws.
Accordingly, the ink jet printer system of the present invention implements a design which overcomes the problem of ink accumulation on the spray head nozzles. The present invention is provided with dual pressure sources, a low volume high pressure constant air source to prevent the accumulation of excess ink on the nozzles, and a high volume low pressure constant air source for drawing the ink from the nozzles for application to the imaging medium.